The present invention relates generally to the field of wireless communications and specifically to methods for controlling the operational mode of mobile terminals in a wireless communication system.
Numerous access schemes exist to allow multiple users to share a communication medium. One such access scheme is known as Code Division Multiple Access (CDMA). In CDMA systems, multiple users share the same carrier frequency and may transmit simultaneously. Each user has its own pseudo-noise (PN) sequence, which is approximately orthogonal to the PN sequences of other users. Transmissions to or from individual users are imprinted with that user's PN sequence. The receiver selects the desired signal, which combines in the communication with unwanted signals, by correlating the received signal with the PN sequence of the desired signal. All other signals are spread by the PN sequence and appear as noise to the receiver.
CDMA systems are interference-limited systems. Since all mobile terminals operate at the same frequency, internal interference generated within the system plays a critical role in determining system capacity and voice quality. The transmit power from each mobile terminal must be controlled to limit interference while maintaining desired performance objectives, e.g., bit error rate (BER), frame error rate (FER), capacity, dropped-call rate, coverage, etc. Normally a mobile terminal transmits a pilot signal continuously on a reverse pilot channel (R-PICH) to enable closed loop power control by the network.
In some implementation of cdma2000, such as 1xEV-DV and 1xEV-DO systems, the forward link is time-multiplexed and transmitted at the full power available to the base station, but with data rates and slot times that vary depending on forward link channel conditions. The data rate that can be supported by the forward link is proportional to the Signal to Noise Ratio (SNR), which changes continuously. The mobile terminal measures the instantaneous SNR of the pilot signal received from each base station in its active set and requests service from the base station providing the strongest signal. The mobile terminal transmits the SNR value, or equivalently the supportable data rate, for the base station providing the strongest signal on a reverse control channel referred to generically herein as the rate control channel.
It has been proposed to reduce interference and hence increase system capacity by introducing a control hold state for mobile terminals with low transmit activity factors. In the control hold state, the mobile terminal suspends or reduces transmissions on many of the reverse link channels, such as a reverse rate control channel or reverse pilot channel. Gating or suspending transmission on the reverse link channels reduces interference, thus increasing the reverse link throughput and capacity. It also results in lower power consumption at the mobile terminal and thus increased battery life.
One method for identifying which mobile terminals should be placed in a control hold state exploits the phenomenon known as temporal locality. Communications to and from a particular user tend to be grouped in time. A mobile terminal that has very recently sent or received a transmission is more likely to receive or send another transmission in the near term, than is a mobile terminal that has not sent or received a transmission for some time. Following this principle, mobile terminals that have recently successfully received or sent transmissions are maintained in an active mode, and those to and from whom no transmissions have been directed for a predetermined time are commanded to enter a control hold state. A straightforward manner of implementing this approach is to maintain countdown timers for each transmission direction, which are loaded with a predetermined value and started upon each successful transmission. Such timers are referred to herein as the forward link inactivity timer and the reverse link inactivity timer. If an inactivity timer counts down to zero before another transmission in its respective direction, it is said to have expired, or timed-out. If both the forward link and the reverse link inactivity timers for a particular mobile terminal have expired, and if there is no data queued awaiting transmission to that mobile terminal, the network may command the mobile terminal to transition to the control hold state. Traditionally, a base station controller (BSC) in the network maintains and manages both inactivity timers, and controls the operational modes of the mobile terminals.
In some cdma2000 networks, such as 1xEV-DV and 1xEV-DO systems, the forward link is a time-shared channel and the base station transmits to only one mobile terminal at any given time. In systems of this type, a scheduler is usually maintained at each base station to schedule forward link transmissions for all mobile terminals served by that base station. When the serving base station schedules data packets for transmission to a mobile terminal, however, the forward link inactivity timer maintained at the BSC may not be started or reset at the proper time due to scheduling delays at the serving base station, or signaling delays between the base station and the BSC, and thus may not accurately reflect the timing of the last forward link transmission to the mobile terminal. This problem may be further complicated by the fact that, in soft handoff, the mobile terminal may select a different base station in its active set as its forward link serving base station, based on received signal quality. This further complicates the synchronization of message scheduling to the mobile terminal at the base station and the state of the forward link inactivity timer at the base station controller.